Typically, a liquid crystal display (LCD) for handheld and notebook devices generally employs at least one lateral light emitting diode (LED) as a light source of a backlight unit. Such a lateral LED is generally provided to the backlight unit as shown in FIG. 1 of Yang U.S. Pat. No. 7,350,958.
Referring to FIG. 1, the backlight unit 10 comprises a planar light guide film 20 disposed on a substrate 12, and a plurality of lateral LEDs 30 (only one lateral LED is shown in FIG. 1) disposed in an array on a lateral side of the light guide film 20. Light L entering the light guide film 20 from the LED 30 is reflected upwardly by a minute reflection pattern 22 and a reflection sheet (not shown) positioned on the bottom of the light guide film 20, and exits from the light guide film 20, providing back light to an LCD panel 40 above the light guide film 20. Such a backlight unit 20 suffers from a problem as shown in FIG. 2 when light is incident on the light guide film 20 from the LED 30.
As shown in FIG. 2, light L emitted from each LED 30 is refracted toward the light guide film 20 by a predetermined angle θ due to difference in refractive index between media according to Snell's Law when the light L enters the light guide film 20. In other words, even though the light L is emitted at a beam angle of α1 from the LED 30, it is incident on the light guide film 20 at an incidence angle of α2 less than α1. In FIG. 3, such an incidence profile of light L is shown. Therefore, there is a problem of increasing the length (l) of a combined region where beams of light L entered the light guide film 20 from the respective LEDs 30 are combined. In addition, light spots H also called “hot spots” and dark spots D are alternately formed in the region corresponding to the length (l) on the incident plane of the light guide film 20. Each of the light spots H is formed at a location facing the LED 30, and each of the dark spots D is formed between the light spots H.
Since the alternately formed light and dark spots are not desirable for the light guide film, they should be minimized and the length (l) should be shortened as much as possible. For this purpose, it is necessary to increase an angle of light entering the light guide film, that is, an incidence angle of light.
For this purpose, it is suggested to form protrusions on the input surface of the light guide film as shown in FIG. 4. Specifically, a plurality of fine prism-shaped structures 24 or arc-shaped structures (not shown) are formed on a light input surface of a light guide film 20A and light L enters the light guide film at an incidence angle α3 substantially equal to an orientation angle α1 of light emitted from a focal point F of a light source. Thus, if orientation angles al of light beams emitted from the focal point F of the light source are identical, the light L enters the light guide film at an incidence angle α3 wider than the case of FIGS. 2 and 3. However, with this solution, there is some secondary light collimation where the light rays are refracted by the wall of the adjacent prism or arc-shaped structure as shown in FIG. 4. Secondary light collimation from the walls of the adjacent prism structure turns the light ray back on-axis providing less diffusion of the light from the light source as shown in FIG. 4. Thus the continuous prism- or arc-shaped structures on the input surface have limited light diffusing capability.
Therefore an improved input edge design is needed to provide a more uniform surface illumination of the light guide film without sacrficing the efficiency of the backlight system.